Study of transient quantum phases of matter via light-control of dynamical charge correlations

我们经常将量子材料识别为固体系统，其中强电子相互作用与低维度相一致，导致无法通过（准）经典方法解释的现象的出现，例如超导性和电荷顺序。在过去的几十年里，科学界对量子材料的基态进行了广泛的研究，但直到最近，我们才开始开发按需控制材料特性的工具。在这方面，光-物质相互作用可能是控制固体性质的最强大和最灵活的调节旋钮。最近，这种方法的原理证明从光诱导超导到光诱导金属到绝缘体的转变。电荷顺序，即电子密度的自重组，是量子材料的共同特征，它可以采取不同的形式，从远程域到短程和动态（波动）电荷相关。后者重新引起了人们的兴趣，因为它们被认为是理解量子材料的一些技术相关特性（如超导性、向列性和线性电阻率）的基本成分（直到最近才被发现）。这个横跨加拿大的多机构合作，连接INRS (Boschini)，麦吉尔大学（Siwick）和QMI-UBC (Damascelli和Berciu)，以及其他国家和国际合作者，建议使用高强度超短光激发（在可见光到太赫兹范围内）来控制各种量子材料中动态电荷相关性的强度和空间分布。这个项目的重要性是基于这样一种观点，即动态电荷相关性可能是所有量子材料的一个普遍特征，它源于固体内部电子相互作用的特定空间依赖性。事实上，虽然自由空间中两个电子之间的相互作用随距离单调衰减（众所周知的库仑定律），但固体的电子局域化和晶格周期性可能导致有效的非单调库仑势，这可能有利于电子配对和相关。使用精确定制特定系统能量学的光脉冲将使我们能够沿着特定晶体学方向瞬时改变晶格结构，以及调整现场电子-电子筛选，以超快的方式锁定或溶解动态电荷相关性。这项研究不仅为固体中动态电荷相关的微观起源提供了全新的信息，而且还将建立动态电荷相关的光控制，作为一种新的、可推广的方法来根据需要调整量子物质的性质。这个雄心勃勃的项目将通过结合互补的超快技术的独特机会成为可能，例如时间分辨光发射光谱（TR-ARPES -使用Boschini和Damascelli两种不同工作参数的系统），超快电子漫射散射（UEDS - Siwick）和基于自由电子激光的x射线散射（TR-XRS - Boschini）。当TR-ARPES通过动量分辨率获得基本电动力学时，UEDS和TR-XRS绘制声子种群和晶格重建的瞬态演化图。此外，理论支持（Berciu）对于指导实验数据的解释至关重要。这个项目不仅将使人们对量子材料的动力学特性有一个前所未有的了解，而且将在加拿大各地培养超快科学领域的高素质人才。

We often identify quantum materials as solid systems wherein strong electron interactions, in concert with low-dimensionality, lead to the emergence of phenomena unexplainable via (quasi-)classical approaches, such as superconductivity, and charge order. The scientific community has extensively studied the ground state of quantum materials in the past decades, but only very recently have we begun to develop tools for controlling the properties of materials on demand. In this regard, light-matter interaction is probably the most powerful and flexible tuning knob to control the properties of solids. Recent proof of principle demonstrations of this approach span light-induced superconductivity to photoinduced metal-to-insulator transitions.Charge order, i.e. the self-reorganization of the electronic density, is a common feature among quantum materials, and it may assume different forms ranging from long-range domains to short-range and dynamical (fluctuating) charge correlations. The later are of renewed interest as they are believed to be a fundamental ingredient (hidden until very recently) for understanding some of the technological-relevant properties of quantum materials such as superconductivity, nematicity and linear resistivity. This multi-institutional collaboration across Canada connecting INRS (Boschini), McGill University (Siwick) and QMI-UBC (Damascelli and Berciu), as well as other national and international collaborators, propose to use high-intense ultrashort light excitations (in the visible to THz range) to control the strength and spatial distribution of dynamical charge correlations in a variety of quantum materials. The importance of this project is backed by the idea that dynamical charge correlations could be a general feature of all quantum materials, arising from the specific spatial dependence of the electron interactions inside solids. In fact, while the interaction among two electrons in free space decays monotonically with distance (the well-known Coulomb's law), the electron-localization and lattice periodicity of solids may result in an effective non-monotonic Coulomb potential which may favour electron pairing and correlations. The use of light pulses precisely tailored to specific systems' energetics will allow us to transiently change the lattice structure along specific crystallographic directions, as well as tune the on-site electron-electron screening to lock or dissolve dynamical charge correlations in an ultrafast fashion. Not only will this study offer brand-new information on the microscopic origin of dynamical charge correlations in solids, but it will establish the light-control of dynamical charge correlations as a new, generalizable approach to tune the properties of quantum matter on demand.This ambitious project will be made possible by the unique opportunity of combining complementary ultrafast techniques, such as time-resolved photoemission spectroscopy (TR-ARPES - using the two systems of Boschini and Damascelli with different working parameters), ultrafast electron diffuse scattering (UEDS - Siwick), and free-electron-laser-based x-ray scattering (TR-XRS - Boschini). While TR-ARPES accesses fundamental electrodynamics with momentum resolution, UEDS and TR-XRS map the transient evolution of the phonon populations and lattice reconstructions. In addition, theoretical support (Berciu) will be essential to guide the interpretation of experimental data. Not only this project will grant an unprecedented peek into the dynamical properties of quantum materials, but will train highly qualified personnel in ultrafast science across Canada.